1. Field of the Invention
The present invention relates to an ophthalmogical photographing apparatus having an image-capturing device for taking photographic images of an eye to be examined.
2. Description of the Related Art
There is known an ophthalmogical photographing apparatus which obtains an image of an examined eye as an electronic image by an image-capturing device, etc., instead of by conventional silver-halide film.
However, in the above-described conventional example, when an image of an examined eye is input as an electronic image, photographic tolerance (latitude) for a conventional silver-halide film is narrow. Therefore, for example, when an image of the eyegrounds is to be input, it is very difficult to obtain information efficiently from a papilla of the optic nerve (optic disk) having high luminance which contains much information and from the retina having low luminance, in particular, the macula lutea. Furthermore, the reflectance of the eyegrounds differs greatly from person to person, and when a photograph is taken using an inappropriate amount of light with respect to this reflectance, an inefficient image having a small area containing useful information with respect to a predetermined dynamic range is formed.
When a photograph of an image of an examined eye is taken, there are cases in which white flaring occurs at the peripheral portion of the image due to the degree of curvature of the cornea and photographic error. In this case, since information of the high-luminance portion, which is unnecessary for the predetermined dynamic range, is assigned closely, an inefficient image is formed. In a similar manner, a black aperture portion in the peripheral portion of the image is a factor which produces an inefficient image in which unnecessary information is assigned to a low-luminance portion.
Furthermore, in electronic images in general, when the amount of image data is large, a greater storage capacity is required, and processes, such as a replay, are thereafter slow. Some ophthalmogical photographing apparatuses use a technique for taking photographs continuously at very short intervals. However, in this case, a problem arises in that the processing speed per image must be shortened greatly.